1. Field of the Invention
This invention relates to a braking pressure control apparatus for a vehicle such as an automotive vehicle.
2. Description of the Prior Art
In most of automotive antiskid control apparatuses, braking pressures applied to wheel cylinders within brake units are adjusted by electromagnetic valves which control the communication among the wheel cylinders, a master cylinder (a pressure source), and a reservoir.
Japanese published examined patent application 51-6308 discloses an antiskid control apparatus in which communication control electromagnetic valves are driven by pulse currents, and thus the rates of increases and decreases in braking pressures applied to wheel cylinders are variable in response to the duty cycles of the drive pulse currents. The antiskid control apparatus of Japanese patent application 51-6308 can realize gradual increases and decreases in the braking pressures.
The antiskid control apparatus of Japanese patent application 51-6308 has the following problem. The response characteristics of the electromagnetic valves considerably limit the minimal value of a variable range of the duty cycles of the drive pulse currents. As a result, fine adjustment of the braking pressures tends to be difficult under certain conditions.